Well, after all that, I am going to return the panel to Costco. All of the connectors are dinky little metal tubes, the kind you'd use for powering a boom box. (I'm showing my age here.)
So the way I figure it, if the manufacturer of this panel thought that this unit could charge a real 12v battery, they would have included some hardware (maybe alligator clips??) that would attach to the battery terminals. But they didn't.
Bottom line -- I might (or might not) be able to get it to work with enough add-on hardware, but it is not worth the "sunk costs" of the hardware. I can easily return the panel to Costco -- but it is not so easy to return the hardware for a full refund.
